docs: reorganise documentation; derive header version from package.json; release v5.7.2

No protocol or message-protection changes.

The version in the application header was a literal and had fallen behind,
showing v5.6.0 while running 5.7.1. It now comes from package.json, and a test
fails if a hard-coded one reappears or if meta.json, the README badge, the
changelog and the docs disagree about the release.

Documentation reorganised so that everything technical lives in doc/ with an
index, and the root keeps only what belongs there by convention: README,
SECURITY, CHANGELOG and LICENSE.

- SECURITY.md rewritten. It listed a supported release line three major versions
  out of date and made claims the software does not make. It now states what is
  guaranteed, what is not, and how to report a problem.
- SECURITY_DISCLAIMER.md and RESPONSIBLE_USE.md merged into doc/USE-POLICY.md,
  which says what the software cannot protect against rather than listing
  generic advice.
- doc/SECURITY-ARCHITECTURE.md renamed to doc/ARCHITECTURE.md and rewritten
  around the session lifecycle, what verification gates, and how recovery works.
- doc/CRYPTOGRAPHY.md rewritten: key schedule, the Double Ratchet, framing, and
  memory handling, with values taken from the source rather than restated.
- doc/CONFIGURATION.md rewritten with the real file-type policy, ICE and TURN
  guidance, and the deployment caching rules that matter.
- docs/webrtc-config.md moved to doc/CALLS.md and rewritten; the obsolete
  docs/webrtc-audit.md, a working document full of stale line numbers, removed
  along with the docs/ directory.
- doc/CONTRIBUTING.md records what the recent regressions taught us about
  writing tests that can actually fail.
- doc/README.md added as an index.

Internal security review notes are excluded from the repository via .gitignore.
Those describe attack paths against specific releases in enough detail to
reproduce them, which is useful privately and harmful in public while users are
still updating.

src/components/ui/Header.jsx

@@ -1,11 +1,19 @@
-const EnhancedMinimalHeader = ({ 
-    status, 
-    fingerprint, 
-    verificationCode, 
-    onDisconnect, 
-    isConnected, 
-    securityLevel, 
-    webrtcManager 
+// The version shown in the header comes from package.json rather than a literal,
+// so a release cannot ship a header advertising the previous one. It was
+// hard-coded and drifted. A named import lets the bundler inline just this field
+// instead of embedding the whole manifest.
+import { version as packageVersion } from '../../../package.json';
+
+const APP_VERSION = `v${packageVersion}`;
+
+const EnhancedMinimalHeader = ({
+    status,
+    fingerprint,
+    verificationCode,
+    onDisconnect,
+    isConnected,
+    securityLevel,
+    webrtcManager
 }) => {
     const [realSecurityLevel, setRealSecurityLevel] = React.useState(null);
     const [lastSecurityUpdate, setLastSecurityUpdate] = React.useState(0);
@@ -559,7 +567,7 @@ const EnhancedMinimalHeader = ({
                     React.createElement('div', { key: 'txt', style: { lineHeight: 1.2, minWidth: 0 } }, [
                         React.createElement('div', { key: 'r1', style: { display: 'flex', alignItems: 'baseline', gap: '7px' } }, [
                             React.createElement('span', { key: 'n', style: { fontSize: '16px', fontWeight: 800, letterSpacing: '-0.3px', color: '#e8e8eb' } }, 'SecureBit'),
-                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, 'v5.6.0')
+                            React.createElement('span', { key: 'v', style: { fontFamily: MONO, fontSize: '10px', fontWeight: 500, color: '#56565e' } }, APP_VERSION)
                         ]),
                         React.createElement('div', { key: 'r2', className: 'hidden sm:block', style: { fontSize: '11px', color: '#6b6b73', fontWeight: 500 } }, 'End-to-end encrypted')
                     ])